Materials required: A box or some other outside container; some good insulating or packing material; an inside container for the kettle, or a lining for the nest in which the kettle is placed; a kettle for holding the food; and a cushion, or pad, of insulating material, to cover the top of the kettle.
For the outside container a tightly built wooden box, such as that shown in Figure 39, is satisfactory. The walls should be thick and of some non-conducting material. An old trunk, a small barrel, or a large butter or lard firkin or tin will serve the purpose. Another possibility is a galvanized iron bucket with a closely fitting cover (this has the advantage of being fire-proof). A shoe box 15 by 15 by 28 inches is convenient in size, since it may be divided into two compartments. It should have a hinged cover and, at the front, a hook and staple, or some other device to hold down the cover tightly; an ordinary clamp window fastener answers this purpose very well. The size of the container, which depends upon the size of the kettle used, should be large enough to allow for at least four inches of packing material all round the nest in which the kettle is placed.
[Illustration: _Fig._ 39.--Completed fireless cooker]
For packing or insulating material a variety of substances may be used. Asbestos and mineral wool are the best, and have the additional advantage that they cannot burn. Ground cork (used in packing Malaga grapes), hay, excelsior, Spanish moss, wool, and crumpled paper may also be used satisfactorily. Of these materials crumpled paper is probably the best, as it is clean and odourless and, if properly packed, will hold the heat better than the others. It is wise to line the box with one thickness of heavy paper or with several thicknesses of newspaper, to make it as air-tight as possible. Asbestos sheeting may be used instead. To pack the container with paper, crush single sheets of newspaper between the hands and pack a layer at least four inches deep over the bottom of the outside container, pounding it in with a heavy stick of wood.
Place the inside container for the cooking kettle or the lining for the inside of the nest in the centre of this layer, and pack more crushed paper about it as solidly as possible. The method of packing with paper is shown in Figure 40. If other material is used it should be packed in a similar way.